An absorption refrigerating machine has heretofore suffered a drawback in that an absorption solution used as working medium is crystallized when its concentration becomes high and its temperature becomes low. Therefore, an absorption refrigerating machine is designed with a margin for a crystallization line.
Generally, in the absorption refrigerating machine, a region in which the absorption solution has a high concentration and a low temperature is an outlet for a concentrated solution in a solution heat exchanger. In order to increase the efficiency of the absorption refrigerating machine, it is necessary to lower the temperature of the concentrated solution at this outlet. Therefore, when the efficiency of the absorption refrigerating machine is increased, the temperature of the concentrated solution at the outlet in the solution heat exchanger is lowered, resulting in the crystallization.
Particularly, since a plate type heat exchanger has a good performance of heat transfer and is compact, it is relatively easy to increase the heat transfer area thereof. Thus, it is important to take measures to prevent crystals from being formed when the efficiency of the plate type heat exchanger is increased.
Heretofore, a multitubular (baffle-type) heat exchanger has mainly been used as the solution heat exchanger for the absorption refrigerating machine. Depending on the absorption cycle, a plurality of heat exchangers, as component devices, including a low-temperature solution heat exchanger, a high-temperature solution heat exchanger, an exhaust-heat-recovery heat exchanger, and the like are combined with each other.
These solution heat exchangers are arranged as independent heat exchangers and connected to each other through pipes for performing desired functions.
Since the multitubular heat exchanger is used as the solution heat exchanger, and the solution heat exchangers installed independently of each other are connected to each other through pipes, the solution heat exchanger has the following drawbacks:                {circle around (1)} The amount of the solution in the heat exchangers and the pipes is large, and the start-up characteristics are poor.        
{circle around (2)} Since the solution heat exchanger is large in size and heavy, a reduction in size and weight is difficult to be carried out and is costly.                {circle around (3)} The solution heat exchanger has complicated piping and requires a long time to be manufactured.        {circle around (4)} The solution heat exchanger has a structure that cannot easily be mass-produced.        
Even if these solution heat exchangers are constituted by plate type heat exchangers, the solution heat exchangers installed independently of each other need to be connected to each other through external pipes. Therefore, the piping of the heat exchangers becomes complicated and costly. Further, the space occupied by the heat exchangers cannot be reduced.
Since the plate type heat exchanger tends to have a pressure loss larger than the multitubular heat exchanger, the plate type heat exchanger is required to have a structure that minimizes the pressure loss caused by the external pipes in order to maximize the heat transfer effect within a limited range of allowable pressure losses in absorption refrigerating cycles.